Perforin and granzyme A release as novel tool to measure NK cell activation in chickens

Natural killer (NK) cells are cytotoxic lymphocytes that are present in the circulation but also in many organs including spleen and gut, where they play an important role in the defense against infections. Interaction of NK cells with target cells leads to degranulation, which results in the release of perforin and granzymes in the direct vicinity of the target cell. Chicken NK cells have many characteristics similar to their mammalian counterparts and based on similarities with studies on human NK cells, surface expression of CD107 was always presumed to correlate with granule release. However, proof of this degranulation or in fact the actual presence of perforin (PFN) and granzyme A (GrA) in chicken NK cells and their release upon activation is lacking. Therefore, the purpose of the present study was to determine the presence of perforin and granzyme A in primary chicken NK cells and to measure their release upon degranulation, as an additional tool to study the function of chicken NK cells. Using human specific antibodies against PFN and GrA in fluorescent and confocal microscopy resulted in staining in chicken NK cells. The presence of PFN and GrA was also confirmed by Western blot analyses and its gene expression by PCR. Stimulation of NK cells with the pectin SPE6 followed by flow cytometry resulted in reduced levels of intracellular PFN and GrA, suggesting release of PFN and GrA. Expression of PFN and GrA reversely correlated with increased surface expression of the lysosomal marker CD107. Finally it was shown that the supernatant of activated NK cells, containing the NK cell granule content including PFN and GrA, was able to kill Escherichia coli. This study correlates PFN and GrA release to activation of chicken NK cells and establishes an additional tool to study activity of cytotoxic lymphocytes in chickens.

Characterization of polarization states of canine monocyte derived macrophages

Macrophages can reversibly polarize into multiple functional subsets depending on their micro-environment. Identification and understanding the functionality of these subsets is relevant for the study of immune‑related diseases. However, knowledge about canine macrophage polarization is still in its infancy. In this study, we polarized canine monocytes using GM-CSF/IFN- γ and LPS towards M1 macrophages or M-CSF and IL-4 towards M2 macrophages and compared them to undifferentiated monocytes (M0). Polarized M1 and M2 macrophages were thoroughly characterized for morphology, surface marker features, gene profiles and functional properties. Our results showed that canine M1-polarized macrophages obtained a characteristic large, roundish, or amoeboid shape, while M2-polarized macrophages were smaller and adopted an elongated spindle-like morphology. Phenotypically, all macrophage subsets expressed the pan-macrophage markers CD14 and CD11b. M1-polarized macrophages expressed increased levels of CD40, CD80 CD86 and MHC II, while a significant increase in the expression levels of CD206, CD209, and CD163 was observed in M2-polarized macrophages. RNAseq of the three macrophage subsets showed distinct gene expression profiles, which are closely associated with immune responsiveness, cell differentiation and phagocytosis. However, the complexity of the gene expression patterns makes it difficult to assign clear new polarization markers. Functionally, undifferentiated -monocytes, and M1- and M2- like subsets of canine macrophages can all phagocytose latex beads. M2-polarized macrophages exhibited the strongest phagocytic capacity compared to undifferentiated monocytes- and M1-polarized cells. Taken together, this study showed that canine M1 and M2-like macrophages have distinct features largely in parallel to those of well-studied species, such as human, mouse and pig. These findings enable future use of monocyte derived polarized macrophages particularly in studies of immune related diseases in dogs.

Effects of Escherichia coli LPS Structure on Antibacterial and Anti-Endotoxin Activities of Host Defense Peptides

The binding of Host Defense Peptides (HDPs) to the endotoxin of Gram-negative bacteria has important unsolved aspects. For most HDPs, it is unclear if binding is part of the antibacterial mechanism or whether LPS actually provides a protective layer against HDP killing. In addition, HDP binding to LPS can block the subsequent TLR4-mediated activation of the immune system. This dual activity is important, considering that HDPs are thought of as an alternative to conventional antibiotics, which do not provide this dual activity. In this study, we systematically determine, for the first time, the influence of the O-antigen and Lipid A composition on both the antibacterial and anti-endotoxin activity of four HDPs (CATH-2, PR-39, PMAP-23, and PMAP36). The presence of the O-antigen did not affect the antibacterial activity of any of the tested HDPs. Similarly, modification of the lipid A phosphate (MCR-1 phenotype) also did not affect the activity of the HDPs. Furthermore, assessment of inner and outer membrane damage revealed that CATH-2 and PMAP-36 are profoundly membrane-active and disrupt the inner and outer membrane of Escherichia coli simultaneously, suggesting that crossing the outer membrane is the rate-limiting step in the bactericidal activity of these HDPs but is independent of the presence of an O-antigen. In contrast to killing, larger differences were observed for the anti-endotoxin properties of HDPs. CATH-2 and PMAP-36 were much stronger at suppressing LPS-induced activation of macrophages compared to PR-39 and PMAP-23. In addition, the presence of only one phosphate group in the lipid A moiety reduced the immunomodulating activity of these HDPs. Overall, the data strongly suggest that LPS composition has little effect on bacterial killing but that Lipid A modification can affect the immunomodulatory role of HDPs. This dual activity should be considered when HDPs are considered for application purposes in the treatment of infectious diseases.

Chimeric Peptidomimetic Antibiotic Efficiently Neutralizes Lipopolysaccharides (LPS) and Bacteria-Induced Activation of RAW Macrophages

Peptide antibiotics have gathered attention given the urgent need to discover antimicrobials with new mechanisms of action. Their extended role as immunomodulators makes them interesting candidates for the development of compounds with dual mode of action. The objective of this study was to test the anti-inflammatory capacity of a recently reported chimeric peptidomimetic antibiotic (CPA) composed of polymyxin B nonapeptide (PMBN) and a macrocyclic β-hairpin motif (MHM). We investigated the potential of CPA to inhibit lipopolysaccharide (LPS)-induced activation of RAW264.7 macrophages. In addition, we elucidated which structural motif was responsible for this activity by testing CPA, its building blocks, and their parent compounds separately. CPA showed excellent LPS neutralizing activity for both smooth and rough LPSs. At nanomolar concentrations, CPA completely inhibited LPS-induced nitric oxide, TNF-α, and IL-10 secretion. Murepavadin, MHM, and PMBN were incapable of neutralizing LPS in this assay, while PMB was less active compared to CPA. Isothermal titration calorimetry showed strong binding between the CPA and LPS with similar binding characteristics also found for the other compounds, indicating that binding does not necessarily correlate with neutralization of LPS. Finally, we showed that CPA-killed bacteria caused significantly less macrophage activation than bacteria killed with gentamicin, heat, or any of the other compounds. This indicates that the combined killing activity and LPS neutralization of CPA can prevent unwanted inflammation, which could be a major advantage over conventional antibiotics. Our data suggests that immunomodulatory activity can further strengthen the therapeutic potential of peptide antibiotics and should be included in the characterization of novel compounds.

Avian surfactant protein (SP)-A2 first arose in an early tetrapod before the divergence of amphibians and gradually lost the collagen domain

The air-liquid interface of the mammalian lung is lined with pulmonary surfactants, a mixture of specific proteins and lipids that serve a dual purpose-enabling air-breathing and protection against pathogens. In mammals, surfactant proteins A (SP-A) and D (SP -D) are involved in innate defence of the lung. Birds seem to lack the SP-D gene, but possess SP-A2, an additional SP-A-like gene. Here we investigated the evolution of the SP-A and SP-D genes using computational gene prediction, homology, simulation modelling and phylogeny with published avian and other vertebrate genomes. PCR was used to confirm the identity and expression of SP-A analogues in various tissue homogenates of zebra finch and turkey. In silico analysis confirmed the absence of SP-D-like genes in all 47 published avian genomes. Zebra finch and turkey SP-A1 and SP-A2 sequences, confirmed by PCR of lung homogenates, were compared with sequenced and in silico predicted vertebrate homologs to construct a phylogenetic tree. The collagen domain of avian SP-A1, especially that of zebra finch, was dramatically shorter than that of mammalian SP-A. Amphibian and reptilian genomes also contain avian-like SP-A2 protein sequences with a collagen domain. NCBI Gnomon-predicted avian and alligator SP-A2 proteins all lacked the collagen domain completely. Both avian SP-A1 and SP-A2 sequences form separate clades, which are most closely related to their closest relatives, the alligators. The C-terminal carbohydrate recognition domain (CRD) of zebra finch SP-A1 was structurally almost identical to that of rat SP-A. In fact, the CRD of SP-A is highly conserved among all the vertebrates. Birds retained a truncated version of mammalian type SP-A1 as well as a non-collagenous C-type lectin, designated SP-A2, while losing the large collagenous SP-D lectin, reflecting their evolutionary trajectory towards a unidirectional respiratory system. In the context of zoonotic infections, how these evolutionary changes affect avian pulmonary surface protection is not clear.

Probing Cell-Surface Interactions in Fungal Cell Walls by High-Resolution 1 H-Detected Solid-State NMR Spectroscopy

Solid-state NMR (ssNMR) spectroscopy facilitates the non-destructive characterization of structurally heterogeneous biomolecules in their native setting, for example, comprising proteins, lipids and polysaccharides. Here we demonstrate the utility of high and ultra-high field ¹ H-detected fast MAS ssNMR spectroscopy, which exhibits increased sensitivity and spectral resolution, to further elucidate the atomic-level composition and structural arrangement of the cell wall of Schizophyllum commune, a mushroom-forming fungus from the Basidiomycota phylum. These advancements allowed us to reveal that Cu(II) ions and the antifungal peptide Cathelicidin-2 mainly bind to cell wall proteins at low concentrations while glucans are targeted at high metal ion concentrations. In addition, our data suggest the presence of polysaccharides containing N-acetyl galactosamine (GalNAc) and proteins, including the hydrophobin proteins SC3, shedding more light on the molecular make-up of cells wall as well as the positioning of the polypeptide layer. Obtaining such information may be of critical relevance for future research into fungi in material science and biomedical contexts.

Chicken cathelicidin-2 promotes NLRP3 inflammasome activation in macrophages

Chicken cathelicidin-2 (CATH-2) as a host defense peptide has been identified to have potent antimicrobial and immunomodulatory activities. Here, we reported the mechanism by which CATH-2 modulates NLRP3 inflammasome activation. Our results show that CATH-2 and ATP as a positive control induced secretion of IL-1β and IL-1α in LPS-primed macrophages but did not affect secretion of IL-6, IL-12 and TNF-α. Furthermore, CATH-2 induced caspase-1 activation and oligomerization of apoptosis-associated speck-like protein containing a carboxy- terminal caspase recruitment domain (ASC), which is essential for NLRP3 inflammasome activation. However, CATH-2 failed to induce IL-1β secretion in Nlrp3^-/-, Asc^-/- and Casp1^-/- macrophages. Notably, IL-1β and NLRP3 mRNA expression were not affected by CATH-2. In addition, CATH-2-induced NLRP3 inflammasome activation was mediated by K⁺ efflux but independent of the P2X7 receptor that is required for ATP-mediated K⁺ efflux. Gene interference of NEK7 kinase which has been identified to directly interact with NLRP3, significantly reduced IL-1β secretion and caspase-1 activation induced by CATH-2. Furthermore, confocal microscopy shows that CATH-2 significantly induced lysosomal leakage with the diffusion of dextran fluorescent signal. Cathepsin B inhibitors completely abrogated IL-1β secretion and caspase-1 activation as well as attenuating the formation of ASC specks induced by CATH-2. These results all indicate that CATH-2-induced activation of NLRP3 inflammasome is mediated by K⁺ efflux, and involves the NEK7 protein and cathepsin B. In conclusion, our study shows that CATH-2 acts as a second signal to activate NLRP3 inflammasome. Our study provides new insight into CATH-2 modulating immune response.

Innate Immune Training of Human Macrophages by Cathelicidin Analogs

Trained innate immunity can be induced in human macrophages by microbial ligands, but it is unknown if exposure to endogenous alarmins such as cathelicidins can have similar effects. Previously, we demonstrated sustained protection against infection by the chicken cathelicidin-2 analog DCATH-2. Thus, we assessed the capacity of cathelicidins to induce trained immunity. PMA-differentiated THP-1 (dTHP1) cells were trained with cathelicidin analogs for 24 hours and restimulated after a 3-day rest period. DCATH-2 training of dTHP-1 cells amplified their proinflammatory cytokine response when restimulated with TLR2/4 agonists. Trained cells displayed a biased cellular metabolism towards mTOR-dependent aerobic glycolysis and long-chain fatty acid accumulation and augmented microbicidal activity. DCATH-2-induced trained immunity was inhibited by histone acetylase inhibitors, suggesting epigenetic regulation, and depended on caveolae/lipid raft-mediated uptake, MAPK p38 and purinergic signaling. To our knowledge, this is the first report of trained immunity by host defense peptides.

Chicken cathelicidin-2 promotes IL-1β secretion via the NLRP3 inflammasome pathway and serine proteases activity in LPS-primed murine neutrophils

Cathelicidins have antimicrobial and immunomodulatory activities. Previous studies have shown that chicken cathelicidin-2 (CATH-2) exerts strong anti-inflammatory activity through LPS neutralization. However, it is still unclear whether other intracellular signaling pathways are involved in CATH-2 immunomodulation. Therefore, the CATH-2-meadiated immune response was investigated in LPS-primed neutrophils. Firstly, inflammatory cytokines release was determined in LPS-primed neutrophils. The results showed that CATH-2 significantly promoted secretion of IL-1β and IL-1α while IL-6 and TNF-α were not affected. IL-1β is the key indicator of inflammasome activation. Next, NLRP3 inflammasome signaling pathway was explored using neutrophils of Nlrp3^-/-, Asc^-/- and Casp1^-/- mice and the results showed that the CATH-2-enhanced IL-1β release was completely abrogated, indicating it is NLRP3-dependent. Moreover, CATH-2 significantly induced activation of caspase-1 and gasdermin D (GSDMD) but did not affect LPS-induced mRNA expression of IL-1β and NLRP3, demonstrating that CATH-2 serves as the second signal activating the NLRP3 inflammasome. Furthermore, CATH-2-mediated IL-1β secretion and caspase-1 activation is dependent on potassium efflux but independent of P2X7R. In addition, other signaling pathways including JNK, ERK and SyK were investigated using different inhibitors and the results showed that these signaling pathway inhibitors partially attenuated CATH-2-enhanced IL-1β secretion, especially the JNK inhibitor. Finally, the role of serine protease in CATH-2-mediated NLRP3 inflammasome activation was investigated in neutrophils and the results showed that serine protease activity is involved in CATH-2-enhanced IL-1β secretion and caspase-1 activation. In conclusion, after LPS priming in neutrophils, CATH-2 can be an agonist of the NLRP3 inflammasome. Our study increases the understanding on immunomodulatory effects of chicken cathelicidins and provides new insight on chicken cathelicidins-mediated immune response.

Synthetic Antibiotic Derived from Sequences Encrypted in a Protein from Human Plasma

Encrypted peptides have been recently found in the human proteome and represent a potential class of antibiotics. Here we report three peptides derived from the human apolipoprotein B (residues 887-922) that exhibited potent antimicrobial activity against drug-resistant Klebsiella pneumoniae, Acinetobacter baumannii, and Staphylococci both in vitro and in an animal model. The peptides had excellent cytotoxicity profiles, targeted bacteria by depolarizing and permeabilizing their cytoplasmic membrane, inhibited biofilms, and displayed anti-inflammatory properties. Importantly, the peptides, when used in combination, potentiated the activity of conventional antibiotics against bacteria and did not select for bacterial resistance. To ensure translatability of these molecules, a protease resistant retro-inverso variant of the lead encrypted peptide was synthesized and demonstrated anti-infective activity in a preclinical mouse model. Our results provide a link between human plasma and innate immunity and point to the blood as a source of much-needed antimicrobials.

The cathelicidin CATH-2 efficiently neutralizes LPS- and E. coli-induced activation of porcine bone marrow derived macrophages

Infectious diseases in pigs cause monetary loss to farmers and pose a zoonotic risk. Therefore, it is important to obtain more porcine specific immunological knowledge as a measure to protect against infectious diseases, for example by exploring immunomodulators that are usable as vaccine adjuvants. Cathelicidins are a class of host defence peptides (HDPs) able to directly kill microbes as well as exert a diverse range of effects on the immune system. The peptides have shown promise as immunomodulatory peptides in many applications, including vaccines. However, it is currently unknown what the precise effect of these peptides is on porcine immune cells and whether peptides of other species might also have a strong immunomodulatory effect on porcine macrophages. Mononuclear bone marrow cells of pigs, aged 5-6 months, were cultured into M1 or M2 macrophages and stimulated with LPS or whole bacteria in the presence of host defence peptides (HDPs). CATH-2 and LL-37 strongly inhibited LPS-induced activation of M1 macrophages, the inhibition of LPS-induced activation of M2 macrophages by HDPs was milder, showing that the peptides have selective effects on different cell types. Upon stimulation with whole bacteria, only CATH-2 could effectively inhibit macrophage activation, showing the potent anti-inflammatory potential of this peptide. These results show that porcine peptides are not necessarily the most active in a porcine system, and that CATH-2 is effective in a porcine system as an anti-inflammatory immune modulator, which can be used, for example, in inactivated pathogen vaccines.

The Interplay between Salmonella and Intestinal Innate Immune Cells in Chickens

Salmonellosis is a common infection in poultry, which results in huge economic losses in the poultry industry. At the same time, Salmonella infections are a threat to public health, since contaminated poultry products can lead to zoonotic infections. Antibiotics as feed additives have proven to be an effective prophylactic option to control Salmonella infections, but due to resistance issues in humans and animals, the use of antimicrobials in food animals has been banned in Europe. Hence, there is an urgent need to look for alternative strategies that can protect poultry against Salmonella infections. One such alternative could be to strengthen the innate immune system in young chickens in order to prevent early life infections. This can be achieved by administration of immune modulating molecules that target innate immune cells, for example via feed, or by in-ovo applications. We aimed to review the innate immune system in the chicken intestine; the main site of Salmonella entrance, and its responsiveness to Salmonella infection. Identifying the most important players in the innate immune response in the intestine is a first step in designing targeted approaches for immune modulation.

Outer Membrane Vesicles Protect Gram-Negative Bacteria against Host Defense Peptides

Host defense peptides (HDPs) are part of the innate immune system and constitute a first line of defense against invading pathogens. They possess antimicrobial activity against a broad spectrum of pathogens. However, pathogens have been known to adapt to hostile environments. Therefore, the bacterial response to treatment with HDPs was investigated. Previous observations suggested that sublethal concentrations of HDPs increase the release of outer membrane vesicles (OMVs) in Escherichia coli. First, the effects of sublethal treatment with HDPs CATH-2, PMAP-36, and LL-37 on OMV release of several Gram-negative bacteria were analyzed. Treatment with PMAP-36 and CATH-2 induced release of OMVs, but treatment with LL-37 did not. The OMVs were further characterized with respect to morphological properties. The HDP-induced OMVs often had disc-like shapes. The beneficial effect of bacterial OMV release was studied by determining the susceptibility of E. coli toward HDPs in the presence of OMVs. The minimal bactericidal concentration was increased in the presence of OMVs. It is concluded that OMV release is a means of bacteria to dispose of HDP-affected membrane. Furthermore, OMVs act as a decoy for HDPs and thereby protect the bacterium. IMPORTANCE Antibiotic resistance is a pressing problem and estimated to be a leading cause of mortality by 2050. Antimicrobial peptides, also known as host defense peptides (HDPs), and HDP-derived antimicrobials have potent antimicrobial activity and high potential as alternatives to antibiotics due to low resistance development. Some resistance mechanisms have developed in bacteria, and complete understanding of bacterial defense against HDPs will aid their use in the clinic. This study provides insight into outer membrane vesicles (OMVs) as potential defense mechanisms against HDPs, which will allow anticipation of unforeseen resistance to HDPs in clinical use and possibly prevention of bacterial resistance by the means of OMVs.

PepBiotics, novel cathelicidin-inspired antimicrobials to fight pulmonary bacterial infections

BACKGROUND

Antimicrobial peptides are considered potential alternatives to antibiotics. Here we describe the antibacterial properties of a family of novel cathelicidin-related (CR-) peptides, which we named PepBiotics, against bacteria typically present in cystic fibrosis (CF) patients.

METHODS

Broth dilution assays were used to determine antibacterial activity of PepBiotics under physiological conditions, as well as development of bacterial resistance against these peptides. Toxicity was tested in mice and cell cultures while molecular interactions of PepBiotics with bacterial membrane components was determined using CD, ITC and LPS/LTA induced macrophage studies.

RESULTS

A relatively small number of PepBiotics remained highly antibacterial against CF-related respiratory pathogens Pseudomonas aeruginosa and Staphylococcus aureus, at high ionic strength and low pH. Interestingly, these PepBiotics also prevented LPS/LTA induced activation of macrophages and was shown to be non-toxic to primary human nasal epithelial cells. Furthermore, both P. aeruginosa and S. aureus were unable to induce resistance against CR-163 and CR-172, two PepBiotics selected for their excellent antimicrobial and immunomodulatory properties. Toxicity studies in mice indicated that intratracheal administration of CR-163 was well tolerated in vivo. Finally, interaction of CR-163 with bacterial-type anionic membranes but not with mammalian-type (zwitterionic lipid) membranes was confirmed using ITC and ³¹P solid state NMR.

CONCLUSIONS

PepBiotics are a promising novel class of highly active antimicrobial peptides, of which CR-163 showed the most potential for treatment of clinically relevant (CF-) pathogens in physiological conditions.

GENERAL SIGNIFICANCE

These observations emphasize the therapeutic potential of PepBiotics against CF-related bacterial respiratory infections.

Activity of Mannose-Binding Lectin on Bacterial-Infected Chickens-A Review

Simple Summary

In the quest to combat bacterial-related diseases in chickens, different methods, of which some are less economical and less effective on the long-term, have been adapted. However, chickens possess mannose-binding lectin (MBL) which could be vital in managing pathogenic bacteria in chickens. MBL is one of the soluble proteins secreted by the chicken’s innate immune system which can be activated when chickens are exposed to chicken-related diseases. This review explains how mannose-binding lectin activation can help in fighting bacterial pathogens in chickens. This knowledge is believed to reduce incessant use of antibiotics and to assist in developing a profitable breeding program with less or no adverse effect on the chicken, human and the environment.

Abstract

In recent years, diseases caused by pathogenic bacteria have profoundly impacted chicken production by causing economic loss in chicken products and by-product revenues. MBL (mannose-binding lectin) is part of the innate immune system (IIS), which is the host’s first line defense against pathogens. The IIS functions centrally by identifying pathogen-specific microorganism-associated molecular patterns (MAMPs) with the help of pattern recognition receptors (PRRs). Studies have classified mannose-binding lectin (MBL) as one of the PRR molecules which belong to the C-type lectin family. The protective role of MBL lies in its ability to activate the complement system via the lectin pathway and there seems to be a direct link between the chicken’s health status and the MBL concentration in the serum. Several methods have been used to detect the presence, the level and the structure of MBL in chickens such as Enzyme-linked immunosorbent assay (ELISA), Polymerase Chain Reaction (PCR) among others. The concentration of MBL in the chicken ranges from 0.4 to 35 µg/mL and can be at peak levels at three to nine days at entry of pathogens. The variations observed are known to depend on the bacterial strains, breed and age of the chicken and possibly the feed manipulation strategies. However, when chicken MBL (cMBL) becomes deficient, it can result in malfunctioning of the innate immune system, which can predispose chickens to diseases. This article aimed to discuss the importance and components of mannose-binding lectin (MBL) in chickens, its mode of actions, and the different methods used to detect MBL. Therefore, more studies are recommended to explore the causes for low and high cMBL production in chicken breeds and the possible effect of feed manipulation strategies in enhancing cMBL production.

Host defence peptides identified in human apolipoprotein B as promising antifungal agents

Abstract

Therapeutic options to treat invasive fungal infections are still limited. This makes the development of novel antifungal agents highly desirable. Naturally occurring antifungal peptides represent valid candidates, since they are not harmful for human cells and are endowed with a wide range of activities and their mechanism of action is different from that of conventional antifungal drugs. Here, we characterized for the first time the antifungal properties of novel peptides identified in human apolipoprotein B. ApoB-derived peptides, here named r(P)ApoB_L^Pro, r(P)ApoB_L^Ala and r(P)ApoB_S^Pro, were found to have significant fungicidal activity towards Candida albicans (C. albicans) cells. Peptides were also found to be able to slow down metabolic activity of Aspergillus niger (A. niger) spores. In addition, experiments were carried out to clarify the mechanism of fungicidal activity of ApoB-derived peptides. Peptides immediately interacted with C. albicans cell surfaces, as indicated by fluorescence live cell imaging analyses, and induced severe membrane damage, as indicated by propidium iodide uptake induced upon treatment of C. albicans cells with ApoB-derived peptides. ApoB-derived peptides were also tested on A. niger swollen spores, initial hyphae and branched mycelium. The effects of peptides were found to be more severe on swollen spores and initial hyphae compared to mycelium. Fluorescence live cell imaging analyses confirmed peptide internalization into swollen spores with a consequent accumulation into hyphae. Altogether, these findings open interesting perspectives to the application of ApoB-derived peptides as effective antifungal agents.

Key points

Human cryptides identified in ApoB are effective antifungal agents.

ApoB-derived cryptides exert fungicidal effects towards C. albicans cells.

ApoB-derived cryptides affect different stages of growth of A. niger.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11114-3.

Outer Membrane Vesicle Induction and Isolation for Vaccine Development

Gram-negative bacteria release vesicular structures from their outer membrane, so called outer membrane vesicles (OMVs). OMVs have a variety of functions such as waste disposal, communication, and antigen or toxin delivery. These vesicles are the promising structures for vaccine development since OMVs carry many surface antigens that are identical to the bacterial surface. However, isolation is often difficult and results in low yields. Several methods to enhance OMV yield exist, but these do affect the resulting OMVs. In this review, our current knowledge about OMVs will be presented. Different methods to induce OMVs will be reviewed and their advantages and disadvantages will be discussed. The effects of the induction and isolation methods used in several immunological studies on OMVs will be compared. Finally, the challenges for OMV-based vaccine development will be examined and one example of a successful OMV-based vaccine will be presented.

The immunomodulatory effect of cathelicidin-B1 on chicken macrophages

Cathelicidins (CATHs) play an important role in the innate immune response against microbial infections. Among the four chicken cathelicidins, CATH-B1 is studied the least. In this study, the effect of CATH-B1 on the macrophage response towards avian pathogenic E. coli (APEC) and bacterial ligands was investigated. Our results show that APEC induced CATH-B1 gene expression in both a chicken macrophage cell line (HD11 cells) and primary macrophages, while expression of the other three CATHs was virtually unaffected. While the antimicrobial activity of CATH-B1 is very low under cell culture conditions, it enhanced bacterial phagocytosis by macrophages. Interestingly, CATH-B1 downregulated APEC-induced gene expression of pro-inflammatory cytokines (IFN-β, IL-1β, IL-6 and IL-8) in primary macrophages. In addition, CATH-B1 pre-incubated macrophages showed a significantly higher gene expression of IL-10 after APEC challenge, indicating an overall anti-inflammatory profile for CATH-B1. Using isothermal titration calorimetry (ITC), CATH-B1 was shown to bind LPS. This suggests that CATH-B1 reduces toll like receptor (TLR) 4 dependent activation by APEC which may partly explain the decreased production of pro-inflammatory cytokines by macrophages. On the contrary, direct binding of CATH-B1 to ODN-2006 enhanced the TLR21 dependent activation of macrophages as measured by nitric oxide production. In conclusion, our results show for the first time that CATH-B1 has several immunomodulatory activities and thereby could be an important factor in the chicken immune response.

A method to differentiate chicken monocytes into macrophages with proinflammatory properties

Macrophages are part of the first line of defense against invading pathogens. In mammals, the in vitro culture of macrophages from blood monocytes or bone marrow cells is well established, including culturing conditions to differentiate them towards M1 or M2-like macrophages. In chicken, monocyte-derived macrophages have been used in several studies, but there is no uniform protocol or actual characterization of these cells. Therefore, to generate proinflammatory M1-like macrophages, in this study blood monocytes were differentiated using GM-CSF for 4 days and characterized based on cell morphology, surface marker expression and cytokine expression response to TLRs stimulation at each (daily) time point. Cell morphology showed that one-day-cultured cells contained a mixture of cell populations, while the homogenous population of cells on day 3 and day 4 were flat and had a 'fried-egg' like shape, similar to human M1 macrophages. In addition, cell surface marker staining showed that 3 and 4- days-cultured cells expressed a high level of MRC1L-B (KUL01) and MHC-II. Furthermore, LPS stimulation of the cultured cells induced gene expression of the proinflammatory cytokines IL-1β, IL-6 and IL-8 after 3 days of culture. Finally, it was shown that day 3 macrophages were able to phagocytose avian pathogenic E. coli (APEC) and respond by nitric oxide production. Overall, our systematic characterization of the monocyte derived cells from blood showed that a 3-days culture was optimal to obtain pro-inflammatory M1 like macrophages, increasing our knowledge about chicken macrophage polarization and providing useful information for studies on chicken macrophage phenotypes.

Cathelicidins Modulate TLR-Activation and Inflammation

Cathelicidins are short cationic peptides that are part of the innate immune system. At first, these peptides were studied mostly for their direct antimicrobial killing capacity, but nowadays they are more and more appreciated for their immunomodulatory functions. In this review, we will provide a comprehensive overview of the various effects cathelicidins have on the detection of damage- and microbe-associated molecular patterns, with a special focus on their effects on Toll-like receptor (TLR) activation. We review the available literature based on TLR ligand types, which can roughly be divided into lipidic ligands, such as LPS and lipoproteins, and nucleic-acid ligands, such as RNA and DNA. For both ligand types, we describe how direct cathelicidin-ligand interactions influence TLR activation, by for instance altering ligand stability, cellular uptake and receptor interaction. In addition, we will review the more indirect mechanisms by which cathelicidins affect downstream TLR-signaling. To place all this information in a broader context, we discuss how these cathelicidin-mediated effects can have an impact on how the host responds to infectious organisms as well as how these effects play a role in the exacerbation of inflammation in auto-immune diseases. Finally, we discuss how these immunomodulatory activities can be exploited in vaccine development and cancer therapies.

Antiviral Activity of Chicken Cathelicidin B1 Against Influenza A Virus

Cathelicidins (CATHs) are host defense peptides (HDPs) that play an important role in the innate immune response against infections. Although multiple functions of cathelicidins have been described, including direct antimicrobial activity and several immunomodulatory effects on the host, relatively little is known about their antiviral activity. Therefore, in vitro antiviral activity of chicken cathelicidins and the underlying mechanism was investigated in this study against different influenza A virus (IAV) strains. Our results show that chicken CATH-B1 has broad anti-IAV activity compared to other cathelicidins (CATH-1, -2, -3, LL-37, PMAP-23, and K9CATH) with an inhibition of viral infection up to 80% against three tested IAV strains (H1N1, H3N1, and H5N1). In agreement herewith, CATH-B1 affected virus-induced inflammatory cytokines expression (IFN-β, IL-1β, IL-6, and IL-8). Incubation of cells with CATH-B1 prior to or after their inoculation with virus did not reduce viral infection indicating that direct interaction of virus with the peptide was required for CATH-B1’s antiviral activity. Experiments using combined size exclusion and affinity-based separation of virus and peptide also indicated that CATH-B1 bound to viral particles. In addition, using electron microscopy, no morphological change of the virus itself was seen upon incubation with CATH-B1 but large aggregates of CATH-B1 and viral particles were observed, indicating that aggregation might be the mechanism of action reducing IAV infectivity. Neuraminidase (NA) activity assays using monovalent or multivalent substrates, indicated that CATH-B1 did not affect NA activity per se, but negatively affected the ability of virus particles to interact with multivalent receptors, presumably by interfering with hemagglutinin activity. In conclusion, our results show CATH-B1 has good antiviral activity against IAV by binding to the viral particle and thereby blocking viral entry.

Chicken CATH-2 Increases Antigen Presentation Markers on Chicken Monocytes and Macrophages

Abstract: Background

Cathelicidins are a family of Host Defense Peptides (HDPs), that play an important role in the innate immune response. They exert both broad-spectrum antimicrobial activity against pathogens, and strong immunomodulatory functions that affect the response of innate and adaptive immune cells.

Objective

The aim of this study was to investigate immunomodulation by the chicken cathelicidin CATH-2 and compare its activities to those of the human cathelicidin LL-37.

Methods

Chicken macrophages and chicken monocytes were incubated with cathelicidins. Activation of immune cells was determined by measuring surface markers Mannose Receptor C-type 1 (MRC1) and MHC-II. Cytokine production was measured by qPCR and nitric oxide production was determined using the Griess assay. Finally, the effect of cathelicidins on phagocytosis was measured using carboxylate-modified polystyrene latex beads.

Results

CATH-2 and its all-D enantiomer D-CATH-2 increased MRC1 and MHC-II expression, markers for antigen presentation, on primary chicken monocytes, whereas LL-37 did not. D-CATH-2 also increased the MRC1 and MHC-II expression if a chicken macrophage cell line (HD11 cells) was used. In addition, LPS-induced NO production by HD11 cells was inhibited by CATH-2 and D-CATH-2.

Conclusion

These results are a clear indication that CATH-2 (and D-CATH-2) affect the activation state of monocytes and macrophages, which leads to optimization of the innate immune response and enhancement of the adaptive immune response.

Studies on citrullinated LL-37: detection in human airways, antibacterial effects and biophysical properties

Arginine residues of the antimicrobial peptide LL-37 can be citrullinated by peptidyl arginine deiminases, which reduce the positive charge of the peptide. Notably, citrullinated LL-37 has not yet been detected in human samples. In addition, functional and biophysical properties of citrullinated LL-37 are not fully explored. The aim of this study was to detect citrullinated LL-37 in human bronchoalveolar lavage (BAL) fluid and to determine antibacterial and biophysical properties of citrullinated LL-37. BAL fluid was obtained from healthy human volunteers after intra-bronchial exposure to lipopolysaccharide. Synthetic peptides were used for bacterial killing assays, transmission electron microscopy, isothermal titration calorimetry, mass-spectrometry and circular dichroism. Using targeted proteomics, we were able to detect both native and citrullinated LL-37 in BAL fluid. The citrullinated peptide did not kill Escherichia coli nor lysed human red blood cells. Both peptides had similar α-helical secondary structures but citrullinated LL-37 was more stable at higher temperatures, as shown by circular dichroism. In conclusion, citrullinated LL-37 is present in the human airways and citrullination impaired bacterial killing, indicating that a net positive charge is important for antibacterial and membrane lysing effects. It is possible that citrullination serves as a homeostatic regulator of AMP-function by alteration of key functions.

Effects of human antimicrobial cryptides identified in apolipoprotein B depend on specific features of bacterial strains

Cationic Host Defense Peptides (HDPs) are endowed with a broad variety of activities, including direct antimicrobial properties and modulatory roles in the innate immune response. Even if it has been widely demonstrated that bacterial membrane represents the main target of peptide antimicrobial activity, the molecular mechanisms underlying membrane perturbation by HDPs have not been fully clarified yet. Recently, two cryptic HDPs have been identified in human apolipoprotein B and found to be endowed with a broad-spectrum antimicrobial activity, and with anti-biofilm, wound healing and immunomodulatory properties. Moreover, ApoB derived HDPs are able to synergistically act in combination with conventional antibiotics, while being not toxic for eukaryotic cells. Here, by using a multidisciplinary approach, including time killing curves, Zeta potential measurements, membrane permeabilization assays, electron microscopy analyses, and isothermal titration calorimetry studies, the antimicrobial effects of ApoB cryptides have been analysed on bacterial strains either susceptible or resistant to peptide toxicity. Intriguingly, it emerged that even if electrostatic interactions between negatively charged bacterial membranes and positively charged HDPs play a key role in mediating peptide toxicity, they are strongly influenced by the composition of negatively charged bacterial surfaces and by defined extracellular microenvironments.

Avian pathogenic Escherichia coli infection of a chicken lung epithelial cell line

Virulent strains of Escherichia coli (Avian Pathogenic E. Coli: APEC) can cause initial infection of the respiratory tract in chickens potentially leading to systemic infection called colibacillosis, which remains a major cause of economic losses in the poultry industry. The role of epithelial lung cells as first targets of APEC and in initiating the innate immune response is unclear and was investigated in this study. APEC was able to adhere and subsequently invade cells from the chicken lung epithelial CLEC213 cell line exhibiting pneumocyte type II-like characteristics. Invasion was confirmed using confocal microscopy after infection with GFP-labelled APEC. Moreover, the infection resulted in a significant increase in IL-8 gene expression, a chemo-attractant of macrophages and heterophils. Gene expression of interferon α and β were not significantly upregulated and chicken Surfactant Protein A, also did not show a significant upregulation on either gene or protein level. The immune response of CLEC213 cells towards APEC was shown to be similar to stimulation with E. coli LPS. These results establish CLEC213 cells as a novel model system for studying bacterial infection of the lung epithelium and show that these cells may play a role in the initial innate response towards bacterial pathogens.

Cathelicidins PMAP-36, LL-37 and CATH-2 are similar peptides with different modes of action

Host defense peptides (HDPs) play a pivotal role in innate immunity and have, in addition to antimicrobial activity, also important immunomodulatory functions. Bacteria are less likely to develop resistance against HDPs because these peptides target and kill bacteria in multiple ways, as well as modulate the immune system. Therefore, HDPs, and derivatives thereof, are promising alternatives to traditional antibiotics. Hardly anything is known about the immunomodulatory functions of porcine cathelicidin PMAP-36. In this study, we aimed to determine both antibacterial and immunomodulatory activities of PMAP-36 comparing the properties of PMAP-36 analogs with two well-studied peptides, human LL-37 and chicken CATH-2. Transmission electron microscopy revealed different killing mechanisms of E. coli for PMAP-36, CATH-2 and LL-37. LL-37 binds LPS very weakly in contrast to PMAP-36, but it inhibits LPS activation of macrophages the strongest. The first 11 amino acids of the N-terminal side of PMAP-36 are dispensable for E. coli killing, LPS-neutralization and binding. Deletion of four additional amino acids resulted in a strong decrease in activity. The activity of full length PMAP-36 was not affected by monomerization, whereas the shorter analogs require dimerization for proper immunomodulatory activity but not for their antibacterial activity.

Antifungal activities of surfactant protein D in an environment closely mimicking the lung lining

At the lung lining innate defenses protect our lungs against inhaled fungal cells that could pose a threat to our health. These defenses are comprised of mucociliary clearance, soluble effector molecules and roaming phagocytic cells, such as macrophages and neutrophils. How important each of these defenses is during fungal clearance depends on the specific fungal pathogen in question and on the stage of infection. In this study the localization and antifungal activity of the lung surfactant protein D (SP-D) was studied in an environment mimicking the lung lining. To this end Calu-3 cells were grown on an air-liquid interface allowing them to polarize and to produce mucus at their apical surface. Additionally, neutrophils were added to study their role in fungal clearance. Two fungal pathogens were used for these experiments: Candida albicans and Aspergillus fumigatus, both of clinical relevance. During fungal infection SP-D localized strongly to both fungal surfaces and stayed bound through the different stages of infection. Furthermore, SP-D decreased fungal adhesion to the epithelium and increased fungal clearance by neutrophils from the epithelial surface. These findings suggest that SP-D plays an important role at the different stages of pulmonary defense against fungal intruders.

A canine keratinocyte cell line expresses antimicrobial peptide and cytokine genes upon stimulation with bacteria, microbial ligands and recombinant cytokines

Keratinocytes (KC) are the main cellular components of the stratum corneum that constitutes a solid physical skin barrier representing the first line of defense against pathogens. Moreover, KC are potent producers of inflammatory mediators and antimicrobial peptides (AMP) when activated through their pattern recognition receptors. In atopic dermatitis (AD) the protective skin barrier may be compromised due to barrier disruption, secondary infection and accelerated secretion of inflammatory cytokines which may also affect AMP expression in the skin. In the present study, we addressed the responses of a canine KC cell line upon exposure to Staphylococcus pseudintermedius, typically found on canine atopic skin during secondary infections, and stimulation by individual AD-associated ligands and cytokines. All stimuli induced a significant increase in expression of the pro-inflammatory cytokine genes tumor necrosis factor (TNF)-α and interleukin (IL)-8, but with different kinetics. Limited effects were observed on AMP gene expression except for K9CATH which was significantly upregulated upon bacterial infection but with none of the individual AD-associated ligands. Interestingly, K9CATH possessed antimicrobial activity towards Staphylococcus pseudintermedius, indicating that K9CATH expression is a specific defense reaction towards bacterial infection and not part of a general pro-inflammatory profile of KC.

The wet bridge transfer system: a novel tool to assess exogenous surfactant as a vehicle for intrapulmonary drug delivery

Due to its branching structure, drug delivery to the peripheral areas of the lung is a major challenge. Consequently, most pulmonary therapies utilize large systemic dosing, with the potential for adverse side effects. One proposed strategy to overcome this challenge is to use exogenous surfactant, a material capable of distributing throughout the lung, as a pulmonary drug delivery vehicle. The objective was to develop and test an in vitro system to rapidly assess surfactant based therapies prior to animal studies. The Wet Bridge Transfer System consisted of two connected wells in which drugs were instilled into a delivery well and function was tested in a remote well which mimicked the remote areas of the lung where drug activity would be required. The system was used to assess surfactant as a carrier for antibiotics (Gentamicin, Ciprofloxacin, and Colistin) by measuring their ability to kill Pseudomonas aeruginosa bacteria in the remote well. Anti-inflammatory agents (Budesonide and a host defense peptide, CATH-2) with and without exogenous surfactant were examined using stimulated macrophages in the remote well and IL-6 concentration as an outcome. The results showed that being paired with surfactant, Gentamicin and Ciprofloxacin, but not Colistin, had significantly greater bacterial killing in the remote wells. Similarly, when combined with a surfactant, both Budesonide and CATH-2 significantly lowered IL-6 concentrations. We conclude that the wet-bridge system can be used to rapidly screen surfactant-based therapies prior to their assessment in vivo. Furthermore, exogenous surfactant was an effective delivery vehicle for several antimicrobial and anti-inflammatory therapeutics.

Avian pathogenic Escherichia coli-induced activation of chicken macrophage HD11 cells

Avian pathogenic Escherichia coli (APEC) can cause severe respiratory diseases in poultry. The initial interaction between APEC and chicken macrophages has not been characterized well and it is unclear how effective chicken macrophages are in neutralizing APEC. Therefore, the effect of APEC on activation of chicken macrophage HD11 cells was studied. Firstly, the effect of temperature (37 vs 41 °C) on phagocytosis of APEC by HD11 cells was determined. The results showed that APEC was more susceptible to being phagocytosed by HD11 cells at 41 °C than 37 °C. Subsequently, the capacity of HD11 cells to kill APEC was shown. In addition, HD11 cells produced nitric oxide (NO) at 18 h post-infection and a strong increase in the mRNA expression of IL-8, IL-6, IL-1β and IL-10 was detected, while IFN-β gene expression remained unaffected. Finally, it was shown that the response of HD11 was partially dependent on viability of APEC since stimulation of HD11 cells with heat-killed APEC resulted in a reduced expression level of these cytokines. In conclusion, APEC induces an effector response in chicken macrophages by enhanced NO production and cytokines gene expression.

A new and efficient culture method for porcine bone marrow-derived M1- and M2-polarized macrophages

BACKGROUND

Macrophages play an important role in the innate immune system as part of the mononuclear phagocyte system (MPS). They have a pro-inflammatory signature (M1-polarized macrophages) or anti-inflammatory signature (M2-polarized macrophages) based on expression of surface receptors and secretion of cytokines. However, very little is known about the culture of macrophages from pigs and more specific about the M1 and M2 polarization in vitro.

METHODS

Porcine monocytes or mononuclear bone marrow cells were used to culture M1- and M2-polarized macrophages in the presence of GM-CSF and M-CSF, respectively. Surface receptor expression was measured with flow cytometry and ELISA was used to quantify cytokine secretion in response to LPS and PAM₃CSK₄ stimulation. Human monocyte-derived macrophages were used as control.

RESULTS

Porcine M1- and M2-polarized macrophages were cultured best using porcine GM-CSF and murine M-CSF, respectively. Cultures from bone marrow cells resulted in a higher yield M1- and M2-polarized macrophages which were better comparable to human monocyte-derived macrophages than cultures from porcine monocytes. Porcine M1-polarized macrophages displayed the characteristic fried egg shape morphology, lower CD163 expression and low IL-10 production. Porcine M2-polarized macrophages contained the spindle-like morphology, higher CD163 expression and high IL-10 production.

CONCLUSION

Porcine M1- and M2-polarized macrophages can be most efficiently cultured from mononuclear bone marrow cells using porcine GM-CSF and murine M-CSF. The new culture method facilitates more refined studies of porcine macrophages in vitro, important for both porcine and human health since pigs are increasingly used as model for translational research.

The Antibacterial and Anti-inflammatory Activity of Chicken Cathelicidin-2 combined with Exogenous Surfactant for the Treatment of Cystic Fibrosis-Associated Pathogens

Cystic fibrosis (CF) is characterized by recurrent airway infections with antibiotic-resistant bacteria and chronic inflammation. Chicken cathelicin-2 (CATH-2) has been shown to exhibit antimicrobial activity against antibiotic-resistant bacteria and to reduce inflammation. In addition, exogenous pulmonary surfactant has been suggested to enhance pulmonary drug delivery. It was hypothesized that CATH-2 when combined with an exogenous surfactant delivery vehicle, bovine lipid extract surfactant (BLES), would exhibit antimicrobial activity against CF-derived bacteria and downregulate inflammation. Twelve strains of CF-pathogens were exposed to BLES+CATH-2 in vitro and killing curves were obtained to determine bactericidal activity. Secondly, heat-killed bacteria were administered in vivo to elicit a pro-inflammatory response with either a co-administration or delayed administration of BLES+CATH-2 to assess the antimicrobial-independent, anti-inflammatory properties of BLES+CATH-2. CATH-2 alone exhibited potent antimicrobial activity against all clinical strains of antibiotic-resistant bacteria, while BLES+CATH-2 demonstrated a reduction, but significant antimicrobial activity against bacterial isolates. Furthermore, BLES+CATH-2 reduced inflammation in vivo when either co-administered with killed bacteria or after delayed administration. The use of a host-defense peptide combined with an exogenous surfactant compound, BLES+CATH-2, is shown to exhibit antimicrobial activity against antibiotic-resistant CF bacterial isolates and reduce inflammation.

Role of Soluble Innate Effector Molecules in Pulmonary Defense against Fungal Pathogens

Fungal infections of the lung are life-threatening but rarely occur in healthy, immunocompetent individuals, indicating efficient clearance by pulmonary defense mechanisms. Upon inhalation, fungi will first encounter the airway surface liquid which contains several soluble effector molecules that form the first barrier of defense against fungal infections. These include host defense peptides, like LL-37 and defensins that can neutralize fungi by direct killing of the pathogen, and collectins, such as surfactant protein A and D, that can aggregate fungi and stimulate phagocytosis. In addition, these molecules have immunomodulatory activities which can aid in fungal clearance from the lung. However, existing observations are based on in vitro studies which do not reflect the complexity of the lung and its airway surface liquid. Ionic strength, pH, and the presence of mucus can have strong detrimental effects on antifungal activity, while the potential synergistic interplay between soluble effector molecules is largely unknown. In this review, we describe the current knowledge on soluble effector molecules that contribute to antifungal activity, the importance of environmental factors and discuss the future directions required to understand the innate antifungal defense in the lung.

Antimicrobial and Immunomodulatory Activity of PMAP-23 Derived Peptides

INTRODUCTION

The Porcine Myeloid Antibacterial Peptide (PMAP)-23 is a porcine host defence peptide with strong antibacterial activity against Gram-positive and Gram-negative bacteria, and fungi.

OBJECTIVE

PMAP-23 and truncated/mutated derivatives were tested for antibacterial and immunomodulatory activities to determine core elements of the peptide required for functionality.

METHODS

PMAP-23 and truncated and/or mutated derivatives were synthesized. Antibacterial activity against Gram positive and negative bacteria was determined using colony counting assays. Cytotoxicity was measured against red blood cells and epithelial cells. Peptide induced cytokine production of epithelial cells was determined by ELISA. LPS neutralization was measured using isothermal titration calorimetry and inhibition of LPS induced cytokine production by macrophages. The effect of peptides on phagocytosis was performed by measuring uptake of fluorescently labelled beads by porcine macrophages.

RESULTS

Truncation of the peptide did not lead to a strong reduction in antibacterial activity, but interestingly, all C-terminal truncated forms were strongly inhibited by salt addition, unlike the full length peptide or the two N-terminally truncated peptides. None of the peptides were hemolytic or toxic in concentrations up to 40 μM. Full length PMAP-23 induced IL-8 production in porcine epithelial cells, however, this activity was lost in all truncated peptides. None of the peptides bound LPS and subsequently did not inhibit LPS-induced cytokine production of monocytes. Finally, all PMAP-23 derived peptides reduced the uptake of beads by freshly isolated monocytes.

CONCLUSION

PMAP-23 is mainly antibacterial with only limited immunomodulating capacity; the full length peptide is required for the full spectrum of activities.

Chicken mannose binding lectin has antiviral activity towards infectious bronchitis virus

Mannose binding lectin (MBL) is a collagenous C-type lectin, which plays an important role in innate immunity. It can bind to carbohydrates on the surface of a wide range of pathogens, including viruses. Here we studied the antiviral effect of recombinant chicken (rc)MBL against Infectious Bronchitis Virus (IBV), a highly contagious coronavirus of chicken. rcMBL inhibited in a dose-dependent manner the infection of BHK-21 cells by IBV-Beaudette, as detected by immunofluorescence staining of viral proteins and qPCR. ELISA and negative staining electron microscopy showed that rcMBL bound directly to IBV, resulting in the aggregation of viral particles. Furthermore, we demonstrated that MBL bound specifically to the spike S1 protein of IBV which mediates viral attachment. This subsequently blocked the attachment of S1 to IBV-susceptible cells in chicken tracheal tissues as shown in protein histochemistry. Taken together, rcMBL exhibits antiviral activity against IBV, based on a direct interaction with IBV virions.

Cathelicidins Inhibit Escherichia coli-Induced TLR2 and TLR4 Activation in a Viability-Dependent Manner

Activation of the immune system needs to be tightly regulated to provide protection against infections and, at the same time, to prevent excessive inflammation to limit collateral damage to the host. This tight regulation includes regulating the activation of TLRs, which are key players in the recognition of invading microbes. A group of short cationic antimicrobial peptides, called cathelicidins, have previously been shown to modulate TLR activation by synthetic or purified TLR ligands and may play an important role in the regulation of inflammation during infections. However, little is known about how these cathelicidins affect TLR activation in the context of complete and viable bacteria. In this article, we show that chicken cathelicidin-2 kills Escherichia coli in an immunogenically silent fashion. Our results show that chicken cathelicidin-2 kills E. coli by permeabilizing the bacterial inner membrane and subsequently binds the outer membrane-derived lipoproteins and LPS to inhibit TLR2 and TLR4 activation, respectively. In addition, other cathelicidins, including human, mouse, pig, and dog cathelicidins, which lack antimicrobial activity under cell culture conditions, only inhibit macrophage activation by nonviable E. coli In total, this study shows that cathelicidins do not affect immune activation by viable bacteria and only inhibit inflammation when bacterial viability is lost. Therefore, cathelicidins provide a novel mechanism by which the immune system can discriminate between viable and nonviable Gram-negative bacteria to tune the immune response, thereby limiting collateral damage to the host and the risk for sepsis.

A new cryptic host defense peptide identified in human 11-hydroxysteroid dehydrogenase-1 β-like: from in silico identification to experimental evidence

BACKGROUND

Host defence peptides (HDPs) are evolutionarily conserved components of innate immunity. Human HDPs, produced by a variety of immune cells of hematopoietic and epithelial origin, are generally grouped into two families: beta structured defensins and variably-structured cathelicidins. We report the characterization of a very promising cryptic human HDP, here called GVF27, identified in 11-hydroxysteroid dehydrogenase-1 β-like protein.

METHODS

Conformational analysis of GVF27 and its propensity to bind endotoxins were performed by NMR, Circular Dichroism, Fluorescence and Dynamic Light Scattering experiments. Crystal violet and WST-1 assays, ATP leakage measurement and colony counting procedures were used to investigate antimicrobial, anti-biofilm, cytotoxicity and hemolytic activities. Anti-inflammatory properties were evaluated by ELISA.

RESULTS

GVF27 possesses significant antibacterial properties on planktonic cells and sessile bacteria forming biofilm, as well as promising dose dependent abilities to inhibit attachment or eradicate existing mature biofilm. It is unstructured in aqueous buffer, whereas it tends to assume a helical conformation in mimic membrane environments as well as it is able to bind lipopolysaccharide (LPS) and lipoteichoic acid (LTA). Notably it is not toxic towards human and murine cell lines and triggers a significant innate immune response by attenuating expression levels of pro-inflammatory interleukins and release of nitric oxide in LPS induced macrophages.

CONCLUSION

Human GVF27 may offer significant advantages as leads for the design of human-specific therapeutics.

GENERAL SIGNIFICANCE

Human cryptic host defence peptides are naturally no immunogenic and for this they are a real alternative for solving the lack of effective antibiotics to control bacterial infections.

Novel human bioactive peptides identified in Apolipoprotein B: Evaluation of their therapeutic potential

Host defence peptides (HDPs) are short, cationic amphipathic peptides that play a key role in the response to infection and inflammation in all complex life forms. It is increasingly emerging that HDPs generally have a modest direct activity against a broad range of microorganisms, and that their anti-infective properties are mainly due to their ability to modulate the immune response. Here, we report the recombinant production and characterization of two novel HDPs identified in human Apolipoprotein B (residues 887-922) by using a bioinformatics method recently developed by our group. We focused our attention on two variants of the identified HDP, here named r(P)ApoB_L and r(P)ApoB_S, 38- and 26-residue long, respectively. Both HDPs were found to be endowed with a broad-spectrum antimicrobial activity while they show neither toxic nor haemolytic effects towards eukaryotic cells. Interestingly, both HDPs were found to display a significant anti-biofilm activity, and to act in synergy with either commonly used antibiotics or EDTA. The latter was selected for its ability to affect bacterial outer membrane permeability, and to sensitize bacteria to several antibiotics. Circular dichroism analyses showed that SDS, TFE, and LPS significantly alter r(P)ApoB_L conformation, whereas slighter or no significant effects were detected in the case of r(P)ApoB_S peptide. Interestingly, both ApoB derived peptides were found to elicit anti-inflammatory effects, being able to mitigate the production of pro-inflammatory interleukin-6 and nitric oxide in LPS induced murine macrophages. It should also be emphasized that r(P)ApoB_L peptide was found to play a role in human keratinocytes wound closure in vitro. Altogether, these findings open interesting perspectives on the therapeutic use of the herein identified HDPs.

Imaging the antimicrobial mechanism(s) of cathelicidin-2

Host defence peptides (HDPs) have the potential to become alternatives to conventional antibiotics in human and veterinary medicine. The HDP chicken cathelicidin-2 (CATH-2) has immunomodulatory and direct killing activities at micromolar concentrations. In this study the mechanism of action of CATH-2 against Escherichia coli (E. coli) was investigated in great detail using a unique combination of imaging and biophysical techniques. Live-imaging with confocal fluorescence microscopy demonstrated that FITC-labelled CATH-2 mainly localized at the membrane of E. coli. Upon binding, the bacterial membrane was readily permeabilized as was shown by propidium iodide influx into the cell. Concentration- and time-dependent effects of the peptide on E. coli cells were examined by transmission electron microscopy (TEM). CATH-2 treatment was found to induce dose-dependent morphological changes in E. coli. At sub-minimal inhibitory concentrations (sub-MIC), intracellular granulation, enhanced vesicle release and wrinkled membranes were observed, while membrane breakage and cell lysis occurred at MIC values. These effects were visible within 1–5 minute of peptide exposure. Immuno-gold TEM showed CATH-2 binding to bacterial membranes. At sub-MIC values the peptide rapidly localized intracellularly without visible membrane permeabilization. It is concluded that CATH-2 has detrimental effects on E. coli at concentrations that do not immediately kill the bacteria.

Developmental regulation of chicken surfactant protein A and its localization in lung

Surfactant Protein A (SP-A) is a collagenous C-type lectin (collectin) that plays an important role in the early stage of the host immune response. In chicken, SP-A (cSP-A) is expressed as a 26 kDa glycosylated protein in the lung. Using immunohistochemistry, cSP-A protein was detected mainly in the lung lining fluid covering the parabronchial epithelia. Specific cSP-A producing epithelial cells, resembling mammalian type II cells, were identified in the parabronchi. Gene expression of cSP-A markedly increased from embryonic day 14 onwards until the time of hatch, comparable to the SP-A homologue chicken lung lectin, while mannan binding lectin and collectins CL-L1 and CL-K1 only showed slightly changed expression during development. cSP-A protein could be detected as early as ED 18 in lung tissue using Western blotting, and expression increased steadily until day 28 post-hatch. Our observations are a first step towards understanding the role of this protein in vivo.

Prophylactic administration of chicken cathelicidin-2 boosts zebrafish embryonic innate immunity

Chicken cathelicidin-2 (CATH-2) is a host defense peptide that exhibits immunomodulatory and antibacterial properties. Here we examined effects of CATH-2 in zebrafish embryos in the absence and presence of infection. Yolk-injection of 0.2-1.5 h post-fertilized (hpf) zebrafish embryos with 2.6 ng/kg CATH-2 increased proliferation of phagocytic cells at 48 hpf by 30%. A lethal infection model was developed to test the prophylactic protective effect of CATH-2 peptide. Embryos (0.2-1.5 hpf) were injected with 2.6 ng/kg CATH-2, challenged with a lethal dose of fluorescently labeled Salmonella enteritidis pGMDs3 at 28 hpf and monitored for survival. Prophylactic treatment with CATH-2 was found to delay infection starting at 22 h post-infection (hpi). At 18-20 hpi, significantly lower (2-fold) fluorescence intensity and decreased bacterial loads were detected in peptide-treated embryos. Thus prophylactic administration of low CATH-2 concentrations confer partial protection in zebrafish embryos by boosting the innate immune system.

Protective effect of in ovo treatment with the chicken cathelicidin analog D-CATH-2 against avian pathogenic E. coli

Increasing antibiotic resistance and ever stricter control on antibiotic use are a driving force to develop alternatives to antibiotics. One such strategy is the use of multifunctional Host Defense Peptides. Here we examined the protective effect of prophylactic treatment with the D analog of chicken cathelicidin-2 (D-CATH-2) against a respiratory E. coli infection. Chickens were treated with D-CATH-2 in ovo at day 18 of embryonic development or intramuscularly at days 1 and 4 after hatch. At 7 days of age, birds were challenged intratracheally with avian pathogenic E. coli. Protection was evaluated by recording mortality, morbidity (Mean Lesion Score) and bacterial swabs of air sacs at 7 days post-infection. In ovo D-CATH-2 treatment significantly reduced morbidity (63%) and respiratory bacterial load (>90%), while intramuscular treatment was less effective. D-CATH-2 increased the percentage of peripheral blood lymphocytes and heterophils by both administration routes. E. coli specific IgM levels were lower in in ovo treated animals compared to intramuscular D-CATH-2 treatment. In short, in ovo treatment with the Host Defense Peptide derived D-CATH-2 can partially protect chickens from E. coli infection, making this peptide an interesting starting point to develop alternatives to antibiotics for use in the poultry sector.

N-Terminal Ile-Orn- and Trp-Orn-Motif Repeats Enhance Membrane Interaction and Increase the Antimicrobial Activity of Apidaecins against Pseudomonas aeruginosa

The Gram-negative bacterium Pseudomonas aeruginosa is a life-threatening nosocomial pathogen due to its generally low susceptibility toward antibiotics. Furthermore, many strains have acquired resistance mechanisms requiring new antimicrobials with novel mechanisms to enhance treatment options. Proline-rich antimicrobial peptides, such as the apidaecin analog Api137, are highly efficient against various Enterobacteriaceae infections in mice, but less active against P. aeruginosa in vitro. Here, we extended our recent work by optimizing lead peptides Api755 (gu-OIORPVYOPRPRPPHPRL-OH; gu = N,N,N′,N′-tetramethylguanidino, O = L-ornithine) and Api760 (gu-OWORPVYOPRPRPPHPRL-OH) by incorporation of Ile-Orn- and Trp-Orn-motifs, respectively. Api795 (gu-O(IO)₂RPVYOPRPRPPHPRL-OH) and Api794 (gu-O(WO)₃RPVYOPRPRPPHPRL-OH) were highly active against P. aeruginosa with minimal inhibitory concentrations of 8–16 and 8–32 μg/mL against Escherichia coli and Klebsiella pneumoniae. Assessed using a quartz crystal microbalance, these peptides inserted into a membrane layer and the surface activity increased gradually from Api137, over Api795, to Api794. This mode of action was confirmed by transmission electron microscopy indicating some membrane damage only at the high peptide concentrations. Api794 and Api795 were highly stable against serum proteases (half-life times >5 h) and non-hemolytic to human erythrocytes at peptide concentrations of 0.6 g/L. At this concentration, Api795 reduced the cell viability of HeLa cells only slightly, whereas the IC₅₀ of Api794 was 0.23 ± 0.09 g/L. Confocal fluorescence microscopy revealed no colocalization of 5(6)-carboxyfluorescein-labeled Api794 or Api795 with the mitochondria, excluding interactions with the mitochondrial membrane. Interestingly, Api795 was localized in endosomes, whereas Api794 was present in endosomes and the cytosol. This was verified using flow cytometry showing a 50% higher uptake of Api794 in HeLa cells compared with Api795. The uptake was reduced for both peptides by 50 and 80%, respectively, after inhibiting endocytotic uptake with dynasore. In summary, Api794 and Api795 were highly active against P. aeruginosa in vitro. Both peptides passed across the bacterial membrane efficiently, most likely then disturbing the ribosome assembly, and resulting in further intracellular damage. Api795 with its IOIO-motif, which was particularly active and only slightly toxic in vitro, appears to represent a promising third generation lead compound for the development of novel antibiotics against P. aeruginosa.

Importance of Endosomal Cathelicidin Degradation To Enhance DNA-Induced Chicken Macrophage Activation

Cathelicidins are essential in the protection against invading pathogens through both their direct antimicrobial activity and their immunomodulatory functions. Although cathelicidins are known to modulate activation by several TLR ligands, little is known about their influence on DNA-induced macrophage activation. In this study, we explored the effects of cathelicidins on DNA-induced activation of chicken macrophages and elucidated the intracellular processes underlying these effects. Our results show that chicken cathelicidin (CATH)-2 strongly enhances DNA-induced activation of both chicken and mammalian macrophages because of enhanced endocytosis of DNA-CATH-2 complexes. After endocytosis, DNA is liberated from the complex because of proteolytic breakdown of CATH-2, after which TLR21 is activated. This leads to increased cytokine expression and NO production. Through the interaction with DNA, CATH-2 can play an important role in modulating the immune response at sites of infection. These observations underline the importance of cathelicidins in sensing bacterial products and regulating immune responses.

Antimicrobial and immunomodulatory activities of PR-39 derived peptides

The porcine cathelicidin PR-39 is a host defence peptide that plays a pivotal role in the innate immune defence of the pig against infections. Besides direct antimicrobial activity, it is involved in immunomodulation, wound healing and several other biological processes. In this study, the antimicrobial- and immunomodulatory activity of PR-39, and N- and C-terminal derivatives of PR-39 were tested. PR-39 exhibited an unexpected broad antimicrobial spectrum including several Gram positive strains such as Bacillus globigii and Enterococcus faecalis. Of organisms tested, only Staphylococcus aureus was insensitive to PR-39. Truncation of PR-39 down to 15 (N-terminal) amino acids did not lead to major loss of activity, while peptides corresponding to the C-terminal part of PR-39 were hampered in their antimicrobial activity. However, shorter peptides were all much more sensitive to inhibition by salt. Active peptides induced ATP leakage and loss of membrane potential in Bacillus globigii and Escherichia coli, indicating a lytic mechanism of action for these peptides. Finally, only the mature peptide was able to induce IL-8 production in porcine macrophages, but some shorter peptides also had an effect on TNF-α production showing differential regulation of cytokine induction by PR-39 derived peptides. None of the active peptides showed high cytotoxicity highlighting the potential of these peptides for use as an alternative to antibiotics.

Campylobacter jejuni is highly susceptible to killing by chicken host defense peptide cathelicidin-2 and suppresses intestinal cathelicidin-2 expression in young broilers

Little is known about the interactions of chicken host defense peptides (HDPs) with Campylobacter jejuni in young chicks. To examine the role of the chicken HDP, cathelicidin-2 (CATH-2) in host-pathogen interactions we challenged 4-day-old Ross 308 broilers with a chicken-derived C. jejuni isolate (WS356) and used the chicken pathogen Salmonella enterica Enteritidis phage type 4 (FGT1) as a reference. Immunohistochemical staining was used to localize CATH-2, C. jejuni and Salmonella enteritidis. Intestinal CATH-2 mRNA expression levels were determined by quantitative PCR. Antibacterial activities of CATH-2 peptide against C. jejuni and S. enteritidis isolates were assessed in colony count assays. In contrast to S. enteritidis, C. jejuni was not seen to attach to intestinal epithelium and C. jejuni challenge did not result in recruitment of CATH-2 containing heterophils to the small intestinal lamina propria. Minimal inhibitory concentrations found for CATH-2 peptide against human- and chicken-derived C. jejuni isolates were similar (0.6-2.5 μM) and much lower than for S. enteritidis (20 μM). Compared to wild-type C. jejuni 81116, the lipooligosaccharide (LOS)-deficient 81116ΔwaaF mutant was much more susceptible to CATH-2. Interestingly, CATH-2 mRNA expression levels in the small intestine were significantly lower 48 h p.i. in C. jejuni-challenged chicks. These findings indicate that human clinical and chicken-derived C. jejuni are equally highly susceptible to chicken CATH-2 peptide and that C. jejuni uses LOS to protect itself to some extent against HDPs. Moreover, suppression of intestinal CATH-2 expression levels may be part of the C. jejuni immune evasion strategy.

Sub-lethal levels of carvacrol reduce Salmonella Typhimurium motility and invasion of porcine epithelial cells

The European ban on the use of antibiotic growth promotors has increased the search for new alternatives to prevent pig intestinal microbial diseases and to stimulate growth. The addition of essential oils or components thereof, such as carvacrol, to pig feed is a promising alternative. In this report we determined the effect of sub-lethal concentrations of carvacrol on Salmonella Typhimurium. At concentrations where growth of Salmonella was not inhibited, carvacrol completely inhibited motility of the bacterium. This loss of motility was not due to the loss of the flagellum or to ATP shortage upon carvacrol treatment. Adhesion of Salmonella to IPEC-J2, porcine intestinal epithelial cells, was not affected by carvacrol but invasion was significantly reduced. In addition, the epithelial gene expression of porcine β-defensin 2, an innate immune response to Salmonella infection, was reduced when Salmonella was exposed to carvacrol. This indicates that invasion but not adhesion of Salmonella triggers the porcine β-defensin 2 expression of porcine epithelial cells.